The present invention generally relates to vehicle brake systems and more particularly relates to a brake power transmission device.
A brake power transmission device of the general type referenced herein is found in EP 705 190 B1. A valve piston of the device is in active connection with a resiliently pretensioned pressure piece which can be moved axially towards a reaction member together with the valve piston. As soon as a predetermined input force is achieved, which overcomes the initial tension of the spring acting upon the pressure piece, said pressure piece stops at the control housing and transmits a portion of the power on the reaction member thus changing the effective surface ratio and at the same time the characteristic booster line. Finally, the pressure piece is in positive engagement with the control housing. This means that the driver, in the operative range above the given input force, has to apply less force with his foot in order to increase the brake power than he would have to apply in the operative are below the given input force. Thus the change of the characteristic line depends on the existing input force (foot force).
The known brake power transmission device offers the driver an uncomfortable functional behavior caused by the fact that the transition between the different characteristic booster lines has a disturbing effect and render the dosage of the brake power more difficult. In case of an emergency braking a certain foot force has always to be exceeded before the brake power transmission device provides a higher boosting ratio. Consequently, valuable time may pass until the predetermined input force has been established. It is important that in case of emergency braking, the brake power is provided quickly and without any time loss because every delay causes a longer stopping distance. On the other hand, the known brake power transmission device also provides the increased boosting force if the input force is only slightly above the given force threshold value and the higher boosting force would not be necessary in principle.
It is the object of the present invention to provide a brake power transmission device enabling a higher boosting only when required by the driver. The device is based on a simple and cost-effective sensing of the brake actuation speed so that a maximum boosting force can be provided immediately after exceeding a predetermined brake actuation speed without any time delay. In other words, the criterion for changing the boosting ratio shall be the actuation speed with which e.g. a brake pedal is actuated. Furthermore the brake power transmission device presents a comfortable operating behavior that can be adjusted particularly well.
According to the invention, this is achieved by detachably connecting the pressure piece to the control housing by means of a coupling element in order to increase boosting on the basis of a relative movement between the input member and the output member or components assigned to it. A considerable advantage of the present invention is that the release criterion is based on the relative displacement speed between valve piston and control housing. Thus the brake power transmission device according to the present invention allows for a detachable coupling between the control housing and the pressure piece if the input member is actuated with high speed thus causing a relative movement between the input member and the output member and/or components assigned to it which the expert in principle is familiar with. Consequently there is the possibility to influence the transmission ratio mechanically on the basis of the actuation speed.
In a favorable embodiment, a coupling element abutting on the control housing is assigned to the pressure piece, said coupling element and engaging into the pressure piece in positive engagement and in a detachable manner. According to the embodiment of the present invention a control device is formed in one piece with the coupling element.
In another embodiment of the present invention a separate control device is provided with a control surface and presents a blocking section which in a first position of the control device engages into a channel for the coupling element and in a second position releases the channel for said coupling element. The functional separation between control (control device) and coupling (coupling element) includes several advantages. On the one hand it offers a particularly reliable function because it excludes that the coupling element jams in an intermediate position. On the other hand the device can be manufactured in a less complicated manner with regard to the tolerances.